A hard carbon anode material and its preparation method in sodium-ion batteries

Abstract

This invention relates to the field of sodium-ion battery technology, and more particularly to a hard carbon anode material, its preparation method, and a sodium-ion battery. The preparation method of the hard carbon anode material includes the following steps: preparing a mixed slurry using biomass materials and phosphate as raw materials; subjecting the mixed slurry to steam explosion treatment to obtain a steam explosion product; subjecting the steam explosion product to oxidative crosslinking treatment to obtain a solidified product; and subjecting the solidified product to carbonization treatment to obtain the hard carbon anode material. This invention achieves chemical anchoring and phosphorus locking through steam explosion treatment, improving the high-temperature phosphorus retention rate. Specifically, during the steam explosion pressure holding stage, phosphate groups undergo in-situ esterification crosslinking with the abundant hydroxyl groups in biomass, forming strong P-O-C bonds. This pre-anchoring effect significantly reduces phosphorus volatilization and loss during high-temperature carbonization, improving phosphorus retention and utilization efficiency.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and in particular to a hard carbon anode material, its preparation method, and a sodium-ion battery. Background Technology

[0002] Hard carbon materials, due to their porous structure, high specific surface area, good chemical stability, and electrical conductivity, have broad application prospects in the energy storage field, especially as anode materials for sodium-ion batteries, solving the problem of traditional graphite materials being unable to adapt to sodium-ion insertion / deintercalation. However, pure hard carbon materials usually suffer from defects such as low initial coulombic efficiency and insufficient electronic conductivity, which limit their further commercial applications.

[0003] To overcome the aforementioned shortcomings, heteroelement doping of hard carbon materials is an effective modification strategy. Phosphorus (P) atoms, due to their large atomic radius and abundant valence electrons, can significantly increase the interlayer spacing of carbon layers after being introduced into the hard carbon framework, facilitating the rapid insertion / extraction of sodium ions. Simultaneously, the lone pair electrons of P atoms can modulate the electron cloud distribution of carbon materials, improving their intrinsic conductivity and adding additional sodium storage active sites. Therefore, phosphorus-doped hard carbon anode materials show great potential in improving the reversible capacity and rate performance of sodium-ion batteries.

[0004] Currently, common methods for preparing phosphorus-doped hard carbon materials mainly include precursor mixing pyrolysis, post-treatment doping, and hydrothermal-assisted methods. The precursor mixing pyrolysis method involves mixing phosphorus-containing compounds (such as phytic acid, phosphoric acid, sodium hypophosphite, etc.) with carbon precursors (such as biomass, resins, sugars, etc.), followed by high-temperature carbonization under an inert atmosphere. This method is relatively simple and easy to operate. The post-treatment doping method involves pre-preparing the hard carbon material and then reacting it with a phosphorus source (such as red phosphorus, white phosphorus vapor, phosphates, etc.) in a gas-phase or solid-phase reaction at high temperature to introduce phosphorus into the carbon material. The hydrothermal-assisted method involves pre-treating the carbon precursor or oxidized hard carbon under hydrothermal conditions using a phosphorus source, followed by pyrolysis.

[0005] The existing preparation processes generally suffer from significant phosphorus loss during high-temperature processing, resulting in low phosphorus retention in the final product. Specifically, in the high-temperature carbonization or heat treatment stage (typically 500–1200°C) required for preparing phosphorus-doped hard carbon, the phosphorus source used (especially red phosphorus, hypophosphite, etc.) or its phosphorus-containing intermediates (such as PH3, P2O5, etc.) have low boiling points or high vapor pressures, making them highly susceptible to escaping from the reaction system in gaseous form. This not only leads to a huge waste of phosphorus sources and increases production costs, but the escaped phosphorus-containing gases are also toxic or corrosive, imposing stringent requirements on the sealing of production equipment and exhaust gas treatment systems, and posing safety and environmental risks.

[0006] Therefore, improving the retention rate of phosphorus in hard carbon anode materials is a technical problem that urgently needs to be solved. Summary of the Invention

[0007] To address the problem of low phosphorus retention in phosphorus-doped hard carbon anode materials in existing technologies, this invention provides a method for preparing hard carbon anode materials. By using steam explosion treatment to achieve chemical anchoring and phosphorus locking, the phosphorus retention rate during high-temperature treatment is improved, thus solving the problem of low phosphorus retention in phosphorus-doped hard carbon anode materials in existing technologies.

[0008] The technical solution adopted by this invention to solve its technical problem is: A method for preparing a hard carbon anode material includes the following steps: A mixture slurry was prepared using biomass materials and phosphates as raw materials. The mixture slurry is subjected to steam explosion treatment to obtain steam explosion products; The steam explosion products are subjected to oxidative crosslinking treatment to obtain a cured product; The cured product is subjected to carbonization treatment to obtain a hard carbon anode material.

[0009] Optionally, the phosphate is an ammonium phosphate.

[0010] Optionally, the phosphate is selected from at least one of ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

[0011] Optionally, the mass ratio of the biomass material to the phosphate is 100:(2.5~15).

[0012] Optionally, the steam pressure of the steam explosion treatment is 1.5~3.5MPa, the steam explosion temperature is 180~240℃, the pressure holding time is 5~30min, and the explosion time is ≤0.1s.

[0013] Optionally, the oxidative crosslinking treatment of the vapor explosion product includes: subjecting the vapor explosion product to oxidative crosslinking curing treatment in an air atmosphere at 200-300°C to obtain a cured product.

[0014] Optionally, carbonizing the cured product includes: carbonizing the cured product under an inert gas at 1200~1400°C to obtain a hard carbon anode material.

[0015] Optionally, the biomass material is selected from at least one of coconut shell, walnut shell, jujube shell, apricot shell, peach shell, bamboo, straw, rice husk, peanut shell, poplar wood, and pine wood.

[0016] Another object of the present invention is to provide a hard carbon anode material, which is prepared by the preparation method of hard carbon anode material as described above.

[0017] Another object of the present invention is to provide a sodium-ion battery comprising the hard carbon anode material as described above.

[0018] The beneficial effects of this invention are: The preparation method of hard carbon anode material provided by this invention achieves chemical anchoring and phosphorus locking through steam explosion treatment, thereby improving the high-temperature phosphorus retention rate. Specifically, during the steam explosion pressure holding stage, phosphate groups undergo in-situ esterification crosslinking with the abundant hydroxyl groups in biomass to form strong POC bonds. This pre-anchoring effect significantly reduces the volatilization and loss of phosphorus during high-temperature carbonization, improves the phosphorus retention rate, and enhances the phosphorus utilization efficiency. Detailed Implementation

[0019] The present invention will now be described in further detail. The embodiments described below are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0020] To address the problem of low phosphorus retention in phosphorus-doped hard carbon anode materials in existing technologies, this invention provides a method for preparing a hard carbon anode material, which includes the following steps: S1: Prepare a mixture slurry using biomass materials and phosphate as raw materials; This step can be performed as follows: The biomass material is crushed and sieved to obtain biomass powder, preferably with a particle size of 20-80 mesh; the biomass powder is mixed with phosphate as a phosphorus source, water is added and stirred evenly, preferably using a high-speed stirrer with a stirring speed of 300-500 rpm and a stirring time of 0.5-2 hours to ensure that the biomass powder and phosphorus source are mixed evenly and to avoid local phosphorus source agglomeration, so as to obtain a mixture slurry; S2: The mixture slurry is subjected to steam explosion treatment to obtain steam explosion products; This step can be performed as follows: The mixture obtained in step S1 is placed in a steam explosion device, and high-pressure steam is introduced for steam explosion treatment; the present invention preferably adopts a catapult-type steam explosion device to ensure instantaneous release of steam energy and achieve effective destruction of the lignin-cellulose structure. In this step, the lignin-cellulose network structure in the biomass is destroyed by steam explosion treatment, the degree of polymerization decreases, and new active sites are formed; at the same time, phosphate, as a phosphorus source, achieves chemical pre-crosslinking with the biomass, and uniformly combines at the molecular scale to obtain steam explosion products. Specifically, during the steam explosion process, lignin-cellulose in biomass undergoes acid-like hydrolysis and thermal degradation, β-O-4 bonds break and depolymerize, reducing the degree of polymerization to 500-2000. At the same time, a large number of active functional groups such as hydroxyl and carboxyl groups are generated on the surface. The instantaneous depressurization not only completely opens the internal pores of the biomass, but also forces the phosphorus-containing solution into the microfibrils and completes chemical bonding. Phosphate and cellulose undergo chemical pre-crosslinking during the steam explosion process to form phosphate ester bonds (POC), thereby achieving pre-anchoring of the phosphorus source through chemical anchoring and phosphorus locking. S3: Oxidative crosslinking treatment is performed on the steam explosion products to obtain the cured products; The present invention preferably includes drying the steam explosion products before the oxidative crosslinking treatment; preferably, the drying temperature is 80~120℃, the drying time is 2~6h, and the drying method is vacuum drying or forced air drying, to ensure that the moisture content of the dried material is ≤5% to avoid moisture affecting the subsequent carbonization effect; In this oxidative crosslinking treatment step, the steam explosion products undergo oxidative crosslinking, which further condenses and cyclizes the POC formed in the steam explosion stage, forming a more stable phosphorus-oxygen-carbon crosslinking network. This significantly improves the phosphorus retention rate during high-temperature carbonization and introduces carboxyl and carbonyl oxygen-containing functional groups at the carbon layer edge, which inhibits carbon layer graphitization, expands interlayer spacing, and promotes closed-cell formation during high-temperature carbonization. S4: Carbonize the cured product to obtain hard carbon anode material.

[0021] The preparation method of hard carbon anode material provided by this invention achieves chemical anchoring and phosphorus locking through steam explosion treatment, thereby improving the high-temperature phosphorus retention rate. Specifically, during the steam explosion pressure holding stage, phosphate groups undergo in-situ esterification crosslinking with the abundant hydroxyl groups in biomass to form strong POC bonds. This pre-anchoring effect significantly reduces the volatilization and loss of phosphorus during high-temperature carbonization, improves the phosphorus retention rate, and enhances the phosphorus utilization efficiency.

[0022] Furthermore, the high temperature and high pressure during the steam explosion process of this invention force the phosphorus-containing solution to penetrate deep into the cell wall of biomass, instantly releasing pressure and breaking the dense structure, thereby achieving a three-dimensional uniform distribution of the phosphorus source at the molecular level and overcoming the defects of surface aggregation and internal sparsity in conventional impregnation methods. By destroying the lignin-cellulose structure of biomass through steam explosion pretreatment, its degree of polymerization is reduced, forming a large number of active sites, achieving uniform binding of the phosphorus source and biomass at the molecular scale, and finally preparing phosphorus-doped hard carbon with uniform phosphorus doping and excellent performance, while simplifying the process, reducing costs, and reducing environmental pollution.

[0023] Furthermore, the present invention preferably uses ammonium phosphate as the phosphate, so that the ammonium phosphate can serve as both a phosphorus source and a nitrogen source. On the one hand, the ammonium phosphate can decompose and release nitrogen- and phosphorus-containing active species during heat treatment, thereby achieving nitrogen / phosphorus co-doping of hard carbon materials. On the other hand, the NH3 released by the ammonium phosphate at high temperature reacts with cellulose chain-severed free radicals / oxygen-containing functional groups to achieve in-situ doping of nitrogen, thereby achieving simultaneous doping of phosphorus and nitrogen.

[0024] When ammonium phosphate is used as the phosphate, the lignin-cellulose network structure in the biomass is destroyed by steam explosion treatment, the degree of polymerization decreases, and new active sites are formed. The ammonium phosphate achieves chemical pre-crosslinking with the biomass. At the same time, the NH3 released at high temperature reacts with the cellulose chain-severed free radicals / oxygen-containing functional groups to achieve in-situ nitrogen doping. The steam explosion products are obtained by uniformly combining at the molecular scale. Furthermore, the phosphate in this invention is preferably selected from at least one of ammonium dihydrogen phosphate and diammonium hydrogen phosphate. Taking advantage of the good solubility of this type of phosphate, it helps to further improve the uniformity of phosphorus and nitrogen distribution in the hard carbon anode material.

[0025] To ensure the overall performance of the hard carbon anode material, the preferred mass ratio of biomass material to phosphate in step S1 of this invention is 100:(2.5~15).

[0026] The preferred conditions for steam explosion treatment in this invention are: steam pressure of 1.5~3.5MPa, steam temperature of 180~240℃, pressure holding time of 5~30min, and explosion time ≤0.1s.

[0027] The present invention preferably involves oxidative crosslinking treatment of the steam explosion products, which includes: oxidative crosslinking curing treatment of the steam explosion products in an air atmosphere at 200-300°C to obtain cured products.

[0028] Specifically, the present invention preferably performs the oxidative crosslinking treatment step according to the following method: In this oxidative crosslinking process, the vapor explosion product is placed in an air atmosphere and heated to 200-300℃ at a rate of 1-3℃ / min for low-temperature oxidative crosslinking and curing treatment for 0.5-1h. This achieves further polycondensation and crosslinking of the POC structure, reduces phosphorus / nitrogen escape during the high-temperature carbonization process, and improves doping efficiency.

[0029] The present invention preferably includes carbonizing the cured product by: carbonizing the cured product under an inert gas at 1200~1400℃ to obtain a hard carbon anode material; the inert gas is preferably selected from at least one of nitrogen and argon, the flow rate of the inert gas is 40~100mL / min, the heating rate is 1~5℃ / min, and the holding time is 1~4h.

[0030] Specifically, the carbonization process can be carried out as follows: the solidified product is placed under inert gas protection, heated to 1200~1400℃ for carbonization treatment, held at the temperature for a certain time and then cooled to room temperature to obtain phosphorus / nitrogen doped hard carbon, i.e. hard carbon anode material; the preferred method of cooling after carbonization treatment in this invention is natural cooling to room temperature to avoid rapid cooling causing cracking of the hard carbon structure.

[0031] The preferred biomass material of this invention is selected from at least one of coconut shell, walnut shell, jujube shell, apricot shell, peach shell, bamboo, straw, rice husk, peanut shell, poplar wood, and pine wood.

[0032] The hard carbon anode material prepared by this invention has a phosphorus doping content of 0.5~3wt%, a nitrogen doping content of 1~5wt%, and a carbon interlayer spacing of 0.37~0.40nm, exhibiting excellent electrochemical performance. As a sodium-ion battery anode material, this hard carbon demonstrates excellent electrochemical performance, with a reversible capacity of 350~380 mAh / g at 0.1C rate. In terms of cycle stability, after 1000 cycles at 5C, the capacity retention rate can still be maintained at 85%.

[0033] In summary, compared with existing technologies, the preparation method of hard carbon anode material provided by this invention has the following advantages: First, the high temperature and high pressure during the steam explosion process force the phosphorus / nitrogen-containing solution to penetrate deep into the cell wall of biomass, and the instantaneous pressure relief breaks the dense structure, achieving a three-dimensional uniform distribution of phosphorus / nitrogen sources at the molecular level. This overcomes the defects of surface aggregation and internal sparsity in conventional impregnation methods, breaks through the limitations of surface impregnation, and achieves deep homogeneous doping. Second, during the steam explosion and pressure holding stage, phosphate groups undergo in-situ esterification and cross-linking with the abundant hydroxyl groups in biomass to form strong POC bonds. This pre-anchoring effect significantly reduces the volatilization and loss of phosphorus during high-temperature carbonization above 1200℃, improves phosphorus utilization efficiency, and makes it possible to precisely control the phosphorus content of 0.5-3 wt%. Third, by using low-temperature oxidative crosslinking, phosphorus / nitrogen locking is enhanced. Compared with crosslinking in an inert atmosphere only, this invention uses an air atmosphere for oxidative crosslinking and curing at 200~300℃. Through the dual anchoring mechanism of primary crosslinking by steam explosion and secondary oxidative crosslinking by air, on the one hand, phosphorus / nitrogen is further locked and phosphorus / nitrogen volatilization is reduced during high-temperature carbonization. On the other hand, oxygen-containing functional groups are introduced, the degree of crosslinking is improved, graphitization is inhibited and closed-cell formation is promoted. Thus, while maintaining high reversible capacity, the initial coulombic efficiency and cycle stability are significantly improved. Fourth, since phosphorus has already bonded into the carbon skeleton before the carbonization step in this invention, after high-temperature carbonization, phosphorus is mainly composed of beneficial PC bonds (expanding interlayer spacing and improving electronic conductivity) and stable PO / P=O bonds (which are conducive to building an inorganic SEI interface rich in Na3PO4), while harmful free phosphorus oxides are negligible, which helps to optimize the occurrence form of phosphorus and improve electrochemical performance. Fifth, the interlayer spacing of the hard carbon anode material is expanded to 0.37~0.40 nm, the reversible capacity at 0.1C can reach 350~380 mAh / g, and the initial coulombic efficiency is improved to 86~90%; the capacity retention rate at high rate (5C) is significantly improved, and the capacity retention rate after 1000 cycles is >85%, and the electrochemical performance is significantly improved.

[0034] Another object of the present invention is to provide a hard carbon anode material, which is prepared by the preparation method of hard carbon anode material as described above.

[0035] The hard carbon anode material provided by this invention achieves chemical anchoring and phosphorus locking through steam explosion treatment during the preparation process, thereby improving the high-temperature phosphorus retention rate. Specifically, during the steam explosion pressure holding stage, phosphate groups undergo in-situ esterification crosslinking with the abundant hydroxyl groups in biomass to form strong POC bonds. This pre-anchoring effect significantly reduces the volatilization and loss of phosphorus during high-temperature carbonization, improves the phosphorus retention rate, and enhances the phosphorus utilization efficiency.

[0036] Another object of the present invention is to provide a sodium-ion battery comprising the hard carbon anode material as described above.

[0037] Specifically, a sodium-ion battery includes a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode is prepared by mixing the hard carbon negative electrode material, binder, and conductive agent as described above in a mass ratio of 80-90:5-10:5-10.

[0038] The sodium-ion battery provided by this invention uses a hard carbon anode material that achieves chemical anchoring and phosphorus locking through steam explosion treatment during the preparation process, thereby improving the high-temperature phosphorus retention rate. Specifically, during the steam explosion pressure holding stage, phosphate groups undergo in-situ esterification cross-linking with the abundant hydroxyl groups in biomass to form strong POC bonds. This pre-anchoring effect significantly reduces the volatilization and loss of phosphorus during high-temperature carbonization, improves the phosphorus retention rate, and enhances the phosphorus utilization efficiency.

[0039] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below.

[0040] Example 1

[0041] This embodiment provides a method for preparing a hard carbon anode material, including the following steps: S1: Raw material pretreatment: Crush and sieve the walnut shells to obtain biomass powder with a particle size of 20-80 mesh; mix the biomass powder with ammonium dihydrogen phosphate at a mass ratio of 100:9, add water and stir evenly at a stirring speed of 400 rpm for 1 hour to obtain a mixture slurry. S2: Steam explosion treatment: The mixture slurry obtained in step S1 is placed into a catapult-type steam explosion device, and high-pressure steam is introduced for steam explosion treatment. The steam explosion steam pressure is 2.5MPa, the steam explosion temperature is 210℃, the pressure holding time is 20min, and the explosion time is ≤0.1s to obtain steam explosion products. S3: Drying treatment: The steam explosion product obtained in step S2 is dried at a temperature of 100℃ for 4 hours using vacuum drying to obtain dried material; the dried material is then placed in an air atmosphere and heated to 250℃ at a rate of 2℃ / min for 1 hour to obtain a cured product. S4: The cured product was heated to 1300℃ under nitrogen protection at a heating rate of 3℃ / min for carbonization treatment. After holding at this temperature for 3 hours, it was naturally cooled to room temperature to obtain hard carbon anode material; wherein the nitrogen flow rate was 70mL / min.

[0042] The hard carbon anode material was tested and found to have a phosphorus doping content of 1.96 wt%, a nitrogen doping content of 2.88 wt%, and a carbon interlayer spacing of 0.376 nm.

[0043] Example 2

[0044] This embodiment provides a method for preparing a hard carbon anode material, including the following steps: S1: Raw material pretreatment: Crush and sieve the walnut shells to obtain biomass powder with a particle size of 20-80 mesh; mix the biomass powder with diammonium hydrogen phosphate at a mass ratio of 100:9, add water and stir evenly at a stirring speed of 400 rpm for 1 hour to obtain a mixture slurry. S2: Steam explosion treatment: The mixture slurry obtained in step S1 is placed into a catapult-type steam explosion device, and high-pressure steam is introduced for steam explosion treatment. The steam explosion steam pressure is 2.5MPa, the steam explosion temperature is 210℃, the pressure holding time is 20min, and the explosion time is ≤0.1s to obtain steam explosion products. S3: Drying treatment: The steam explosion product obtained in step S2 is dried at a temperature of 100℃ for 4 hours using vacuum drying to obtain dried material; the dried material is then placed in an air atmosphere and heated to 250℃ at a rate of 2℃ / min for 1 hour to obtain a cured product. S4: The cured product was heated to 1300℃ under nitrogen protection at a heating rate of 3℃ / min for carbonization treatment. After holding at this temperature for 3 hours, it was naturally cooled to room temperature to obtain hard carbon anode material; wherein the nitrogen flow rate was 70mL / min.

[0045] The hard carbon anode material was tested and found to have a phosphorus doping content of 2.08 wt%, a nitrogen doping content of 4.81 wt%, and a carbon interlayer spacing of 0.381 nm.

[0046] Example 3

[0047] This embodiment provides a method for preparing a hard carbon anode material, including the following steps: S1: Raw material pretreatment: Crush and sieve the coconut shell to obtain biomass powder with a particle size of 20~80 mesh; mix the biomass powder with ammonium dihydrogen phosphate at a mass ratio of 100:15, add water and stir evenly at a stirring speed of 400 rpm for 1 hour to obtain a mixture slurry. S2: Steam explosion treatment: The mixture slurry obtained in step S1 is placed into a catapult-type steam explosion device, and high-pressure steam is introduced for steam explosion treatment. The steam explosion steam pressure is 3.5MPa, the steam explosion temperature is 240℃, the pressure holding time is 5min, and the explosion time is ≤0.1s to obtain steam explosion products. S3: Drying treatment: The steam explosion product obtained in step S2 is dried at a temperature of 100℃ for 4 hours using vacuum drying to obtain dried material; the dried material is then placed in an air atmosphere and heated to 300℃ at a rate of 2℃ / min for low-temperature oxidative crosslinking curing treatment for 0.5 hours to obtain cured product. S4: The cured product was heated to 1400℃ under nitrogen protection at a heating rate of 3℃ / min for carbonization treatment. After holding at this temperature for 3 hours, it was naturally cooled to room temperature to obtain hard carbon anode material; wherein the nitrogen flow rate was 70mL / min.

[0048] The hard carbon anode material was tested and found to have a phosphorus doping content of 2.87 wt%, a nitrogen doping content of 4.59 wt%, and a carbon interlayer spacing of 0.391 nm.

[0049] Example 4

[0050] This embodiment provides a method for preparing a hard carbon anode material, including the following steps: S1: Raw material pretreatment: Crush and sieve the coconut shells to obtain biomass powder with a particle size of 20-80 mesh; mix the biomass powder with ammonium dihydrogen phosphate at a mass ratio of 100:2.5, add water and stir evenly at a stirring speed of 400 rpm for 1 hour to obtain a mixture slurry. S2: Steam explosion treatment: The mixture slurry obtained in step S1 is placed into a catapult-type steam explosion device, and high-pressure steam is introduced for steam explosion treatment. The steam explosion steam pressure is 1.5MPa, the steam explosion temperature is 180℃, the pressure holding time is 30min, and the explosion time is ≤0.1s to obtain steam explosion products. S3: Drying treatment: The steam explosion product obtained in step S2 is dried at a temperature of 100℃ for 4 hours using vacuum drying to obtain dried material; the dried material is then placed in an air atmosphere and heated to 200℃ at a rate of 2℃ / min for 1 hour to obtain a cured product. S4: The cured product was heated to 1200℃ under nitrogen protection at a heating rate of 3℃ / min for carbonization treatment. After holding at this temperature for 3 hours, it was naturally cooled to room temperature to obtain hard carbon anode material; wherein the nitrogen flow rate was 70mL / min.

[0051] The hard carbon anode material was tested and found to have a phosphorus doping content of 0.58 wt%, a nitrogen doping content of 1.04 wt%, and a carbon interlayer spacing of 0.373 nm.

[0052] Each comparative example in this invention is compared with Example 1.

[0053] Comparative Example 1 This comparative example provides a method for preparing a hard carbon anode material, including the following steps: S1: Raw material pretreatment: Crush and sieve the walnut shells to obtain biomass powder with a particle size of 20-80 mesh; mix the biomass powder with ammonium dihydrogen phosphate at a mass ratio of 100:9, add water and stir evenly at a stirring speed of 400 rpm for 1 hour to obtain a mixture slurry. S2: The mixture slurry obtained in step S1 is dried at a temperature of 100℃ for 4 hours using vacuum drying to obtain dried material; the dried material is then placed in an air atmosphere and heated to 250℃ at a rate of 2℃ / min for 1 hour to obtain a cured product. S3: The cured product was heated to 1300℃ under nitrogen protection at a heating rate of 3℃ / min for carbonization treatment. After holding at this temperature for 3 hours, it was naturally cooled to room temperature to obtain hard carbon anode material; wherein the nitrogen flow rate was 70mL / min.

[0054] The hard carbon anode material was tested and found to have a phosphorus doping content of 0.42 wt%, a nitrogen doping content of 1.22 wt%, and a carbon interlayer spacing of 0.368 nm.

[0055] Comparative Example 2 This comparative example provides a method for preparing a hard carbon anode material, including the following steps: S1: Raw material pretreatment: Crush and sieve the walnut shells to obtain biomass powder with a particle size of 20-80 mesh; mix the biomass powder with ammonium dihydrogen phosphate at a mass ratio of 100:9, add water and stir evenly at a stirring speed of 400 rpm for 1 hour to obtain a mixture slurry. S2: Steam explosion treatment: The mixture slurry obtained in step S1 is placed into a catapult-type steam explosion device, and high-pressure steam is introduced for steam explosion treatment. The steam explosion steam pressure is 2.5MPa, the steam explosion temperature is 210℃, the pressure holding time is 20min, and the explosion time is ≤0.1s to obtain steam explosion products. S3: Drying treatment: The steam explosion product obtained in step S2 is dried at a temperature of 100℃ for 4 hours using vacuum drying to obtain dried material; the dried material is then heated to 1300℃ under nitrogen protection at a heating rate of 3℃ / min for carbonization treatment, held at this temperature for 3 hours, and then naturally cooled to room temperature to obtain hard carbon anode material; the nitrogen flow rate is 70mL / min.

[0056] The hard carbon anode material was tested and found to have a phosphorus doping content of 0.96 wt%, a nitrogen doping content of 2.09 wt%, and a carbon interlayer spacing of 0.363 nm.

[0057] Comparative Example 3 This comparative example provides a method for preparing a hard carbon anode material, including the following steps: S1: Raw material pretreatment: Crush and sieve the walnut shells to obtain biomass powder with a particle size of 20-80 mesh; mix the biomass powder with ammonium dihydrogen phosphate at a mass ratio of 100:2, add water and stir evenly at a stirring speed of 400 rpm for 1 hour to obtain a mixture slurry. S2: Steam explosion treatment: The mixture slurry obtained in step S1 is placed into a catapult-type steam explosion device, and high-pressure steam is introduced for steam explosion treatment. The steam explosion steam pressure is 2.5MPa, the steam explosion temperature is 210℃, the pressure holding time is 20min, and the explosion time is ≤0.1s to obtain steam explosion products. S3: Drying treatment: The steam explosion product obtained in step S2 is dried at a temperature of 100℃ for 4 hours using vacuum drying to obtain dried material; the dried material is then placed in an air atmosphere and heated to 250℃ at a rate of 2℃ / min for 1 hour to obtain a cured product. S4: The cured product was heated to 1300℃ under nitrogen protection at a heating rate of 3℃ / min for carbonization treatment. After holding at this temperature for 3 hours, it was naturally cooled to room temperature to obtain hard carbon anode material; wherein the nitrogen flow rate was 70mL / min.

[0058] The hard carbon anode material was tested and found to have a phosphorus doping content of 0.37 wt%, a nitrogen doping content of 0.63 wt%, and a carbon interlayer spacing of 0.370 nm.

[0059] Comparative Example 4 This comparative example provides a method for preparing a hard carbon anode material, including the following steps: S1: Raw material pretreatment: Crush and sieve the walnut shells to obtain biomass powder with a particle size of 20-80 mesh; mix the biomass powder with ammonium dihydrogen phosphate at a mass ratio of 100:20, add water and stir evenly at a stirring speed of 400 rpm for 1 hour to obtain a mixture slurry. S2: Steam explosion treatment: The mixture slurry obtained in step S1 is placed into a catapult-type steam explosion device, and high-pressure steam is introduced for steam explosion treatment. The steam explosion steam pressure is 2.5MPa, the steam explosion temperature is 210℃, the pressure holding time is 20min, and the explosion time is ≤0.1s to obtain steam explosion products. S3: Drying treatment: The steam explosion product obtained in step S2 is dried at a temperature of 100℃ for 4 hours using vacuum drying to obtain dried material; the dried material is then placed in an air atmosphere and heated to 250℃ at a rate of 2℃ / min for 1 hour to obtain a cured product. S4: The cured product was heated to 1300℃ under nitrogen protection at a heating rate of 3℃ / min for carbonization treatment. After holding at this temperature for 3 hours, it was naturally cooled to room temperature to obtain hard carbon anode material; wherein the nitrogen flow rate was 70mL / min.

[0060] The hard carbon anode material was tested and found to have a phosphorus doping content of 3.62 wt%, a nitrogen doping content of 5.96 wt%, and a carbon interlayer spacing of 0.395 nm.

[0061] In all the above embodiments and comparative examples, the phosphorus doping amount was detected according to the following method: The mass percentage of phosphorus (P) in hard carbon materials was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES) after digesting the hard carbon product.

[0062] Nitrogen doping levels were measured using the following method: The percentage of nitrogen in hard carbon was determined using an oxygen-nitrogen-hydrogen analyzer (ONH analyzer).

[0063] The carbon interlayer spacing was measured using the following method: Data patterns of hard carbon materials were acquired using an X-ray diffractometer (XRD). Based on the (002) diffraction peak, the interlayer spacing d002 was calculated using the Bragg equation.

[0064] Sodium-ion batteries were prepared using the negative electrode materials from the above embodiments and comparative examples according to the following method, and their performance was tested: The electrode slurry was prepared by mixing negative electrode material, conductive agent SP, and binder CMC in a ratio of 80:10:10 (mass ratio). Appropriate amounts of ethanol and water were added to form the slurry, which was then coated to a thickness of 100 μm. After drying and pressing, the slurry was fabricated into an electrode sheet. A 2430 coin cell was assembled using sodium metal as the counter electrode, a 1 mol / L NaPF6 DEC / EC (1:1) solution as the electrolyte, and a glass fiber filter paper membrane GF / C as the separator. Its charge-discharge performance was tested at a current density of 0.1C.

[0065] The test results are shown in Table 1.

[0066] Table 1 As can be seen from the data in the table above, the hard carbon anode materials prepared in the various embodiments of the present invention exhibit excellent electrochemical performance.

[0067] The difference between Comparative Example 1 and Example 1 is that no steam explosion treatment was performed. As a result, the ammonium dihydrogen phosphate was not fully combined with the biomass raw materials, resulting in poor oxidative cross-linking effect. During the high-temperature sintering process, more phosphorus and nitrogen elements escaped, the doping amount was significantly reduced, the number of active sites in hard carbon decreased, and the capacity was significantly reduced.

[0068] The difference between Comparative Example 2 and Example 1 is that no oxidative crosslinking treatment was performed. Because the phosphorus element did not form a stable POC chemical bond with the oxygen and carbon atoms in the raw material, more phosphorus and nitrogen elements escaped during the high-temperature sintering process, causing the interlayer spacing to collapse and adversely affecting the comparative capacity and cycle performance.

[0069] The difference between Comparative Example 3 and Example 1 is that the amount of phosphorus source added was reduced. Because the initial amount of phosphorus and nitrogen added was less, the sodium storage sites in the prepared hard carbon material were reduced, resulting in a decrease in specific capacity.

[0070] The difference between Comparative Example 4 and Example 1 is that the amount of phosphorus source added was increased. Because phosphorus and nitrogen elements have a pore-forming effect during carbonization, the surface pore structure of the hard carbon material is more abundant, and the side reactions increase, thereby reducing the initial efficiency and cycle capacity retention rate.

[0071] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for preparing a hard carbon anode material, characterized in that, Includes the following steps: A mixture slurry was prepared using biomass materials and phosphates as raw materials. The mixture slurry is subjected to steam explosion treatment to obtain steam explosion products; The steam explosion products are subjected to oxidative crosslinking treatment to obtain a cured product; The cured product is subjected to carbonization treatment to obtain a hard carbon anode material.

2. The method for preparing the hard carbon anode material as described in claim 1, characterized in that, The phosphate is an ammonium phosphate.

3. The method for preparing the hard carbon anode material as described in claim 1, characterized in that, The phosphate is selected from at least one of ammonium dihydrogen phosphate and diammonium hydrogen phosphate.

4. The method for preparing the hard carbon anode material as described in claim 1, characterized in that, The mass ratio of the biomass material to the phosphate is 100:(2.5~15).

5. The method for preparing the hard carbon anode material as described in claim 1, characterized in that, The steam pressure for the steam explosion treatment is 1.5~3.5MPa, the steam explosion temperature is 180~240℃, the pressure holding time is 5~30min, and the explosion time is ≤0.1s.

6. The method for preparing the hard carbon anode material according to any one of claims 1-5, characterized in that, The oxidative crosslinking treatment of the gas explosion product includes: subjecting the gas explosion product to oxidative crosslinking curing treatment in an air atmosphere at 200~300°C to obtain a cured product.

7. The method for preparing the hard carbon anode material according to any one of claims 1-5, characterized in that, The carbonization treatment of the cured product includes: carbonizing the cured product under an inert gas at 1200~1400°C to obtain a hard carbon anode material.

8. The method for preparing the hard carbon anode material according to any one of claims 1-5, characterized in that, The biomass material is selected from at least one of coconut shell, walnut shell, jujube shell, apricot shell, peach shell, bamboo, straw, rice husk, peanut shell, poplar wood, and pine wood.

9. A hard carbon anode material, characterized in that, The hard carbon anode material is prepared by the preparation method described in any one of claims 1-8.

10. A sodium-ion battery, characterized in that, Including the hard carbon anode material as described in claim 9.

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